1. Field of the Invention
The present invention relates to a semiconductor device, a manufacturing method thereof, and a memory circuit. In particular, the present invention relates to a semiconductor device having an SOI (Silicon On Insulator) structure, a manufacturing method thereof, and a memory circuit.
2. Description of the Background Art
In a MOS transistor having an SOI structure, conventionally, a buried oxide film is formed on a silicon substrate and a thin-film silicon layer is formed on the buried oxide film as a semiconductor layer. Then, a source region, a body region and a drain region are formed in the thin-film silicon layer. Further, a gate electrode is formed on the body region between the source region and the drain region via a gate insulating film. Thus, the MOS transistor is configured. Herein, when the body region is of a P type and the source region and the drain region are of an N type, the MOS transistor is of an N channel type.
In the MOS transistor having an SOI structure, the buried oxide film makes contact with the source region and the drain region in order to decrease parasitic capacitances of the source region and the drain region. Further, the thin-film silicon layer is removed from the periphery of the MOS transistor, and the periphery of the MOS transistor is covered with an element separation oxide film. Japanese Patent Application Laid-Open Nos. 2003-332579 and 2003-197634 disclose an invention concerning the aforementioned MOS transistor having an SOI structure.
In the conventional MOS transistor having an SOI structure, since the P-type body region is sandwiched between the N-type source region and the N-type drain region, PN junction is achieved in this portion. Besides, since the buried oxide film is provided on the bottom face of the body region, the potential of the body region is maintained at a floating state.
Consequently, the conventional MOS transistor having an SOI structure has serious problems in its operation such as a kink phenomenon of a drain current called a body floating effect, decrease of a source-drain withstand voltage, dependence of circuit operation time on operating frequency (history effect) and other phenomena.